Some people like to climb big granite walls, others frozen waterfalls. Photographer Carsten Peter and his climbing partner Chris Heinlein like to climb erupting hornitos, the sharp, extremely steep hollow pinnacles that sometimes form around active vents. "You can't compare it to normal rock," says Peter of a hornito, or spatter cone. "It's very fragile and very unstable. The cone was rumbling and vibrating, but a rock plate was directing the lava in the other direction." His advice: "Don't try this at home."

Though Ol Doinyo Lengai's fresh natrocarbonatite lavas—which erupt at around 1,000 degrees Fahrenheit (500 degrees Celsius)—are roughly half the temperature of more common basalt lavas, the spattering drops of molten rock will still burn through a cotton jumpsuit like a cigarette through nylons. Yet the dramatic mountain in a remote, little-visited corner of Tanzania invites close inspection, especially from volcanologists. The late photographer and renowned volcano chaser Katia Krafft was captivated by what she called the "toy volcano" because its diminutive flows are cool enough to collect with a spoon.

"It's a perfect little laboratory volcano," agrees Barry Dawson of the University of Edinburgh, the first to study the strange lavas, in 1960, for the Tanzania Geological Survey. "The shape and style of the extrusions exactly mimic the flows of basaltic volcanos, except the latter are so much bigger."

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

Unlike other volcanoes, which spew silica-rich basalts, Ol Doinyo Lengai is the only active volcano on Earth that erupts with a carbonatite, a type of igneous rock rich in carbonates such as calcite and dolomite. Usually carbonatites are intruded into other formations and rarely reach Earth's surface as eruptions.

Ol Doinyo Lengai is unusual not only because its volcanic activity brings a carbonatite to the surface, but also because the type of carbonatite is special. Called natrocarbonatite, Ol Doinyo Lengai igneous rock is rich in sodium.

Although in the geologic past there have been other carbonatite volcanoes, it is difficult to tell whether any spewed natrocarbonatite lava. When exposed to moisture, natrocarbonatite quickly transforms in composition and texture and, being extremely water soluble, is easily washed away by rains.

—Mary Jennings

The Ol Doinyo Lengai Websiteit.stlawu.edu/~cnya/A comprehensive website that includes photographs and information on past eruptions, brought to you by Ol Doinyo Lengai expert Celia Nyamweru.